The electronics industry manufactures disk heads, also known as sliders, or tape heads, collectively referred to herein as heads, which are part of a storage-drive assembly. The heads facilitate reading and writing of information to or from a storage media, such as a computer disc or video-cassette-recorder tape. FIG. 1 is a perspective view of a head, not drawn to scale. In a simple magnetic-disc-drive assembly, the head 102 is mounted on an arm above a platter. The platter is a magnetic medium and the head 102 is a transducer. The head 102 in conjunction with the spinning platter converts electrical signals from a computer, television, or camcorder, for example, into an electromagnetic field for reading or writing the information to or from the platter.
Essentially, a head interacts with the storage-drive assembly to induce an energy field. Many different schemes using a head can produce the necessary field to read from and write to a particular media. For instance, a magneto-optical drive assembly uses temperature to change polarity of magneto particles to store information, and a storage-drive assembly incorporating phase-change technology uses a laser to heat the recording layer of the media to change the structure between crystalline and non-crystalline states. As the head's function is primarily as a transducer within these assemblies, cosmetic non-uniformities do not necessarily affect function.
Consequently, without appreciably affecting functionality of the storage-drive assembly, the heads can have several non-uniformities, such as chips 124, which are areas of missing material, and can contain identifying information, such as serial numbers 126, as illustrated in FIG. 2. FIG. 2 illustrates the letter-end surface 128 of the head 102, not drawn to scale.
Manufacturers define differently which non-uniformities are unacceptable and, therefore, constitute a defect. Generally, however, manufacturers typically allow some non-uniformities in non-functional regions, regions without pattern deposited material, while restricting almost all non-uniformities in functional regions, such as copper coils 110, photoresist insulators 118, pole-tips 112, leads 116, and pads 114. For instance, cracks 132 and 122 may constitute defects depending upon the location of the cracks 132 and 122. Cracks are breaks in the material of the head, and are typically two microns or more across on a head of typically a few millimeters in area.
One common defect is a crack 120 that touches two edges of the head. Cracks that extend to the edge 130 of the head 102 can grow during operation of the assembly. A grown crack 120 can cause portions of the head 102 to break and fall onto parts of the storage-drive assembly rendering it inoperable or damaging the information stored therein. As such, heads containing these cracks 120 typically must be removed before they are assembled in a storage-drive assembly.
Unlike other typical defects, which are routinely found on the head 102 using machine vision, such as contamination, delamination, and incomplete pads, detecting the cracks 120, 132, 122 typically has proven difficult to automate. The difficulties arise because the textured surface of heads provides an over abundance of directional information making the identification of the cracks 120, 132, 122 difficult. In addition, difficulty also arises because not all cracks on the surface require the head to be discarded. Removing a head with only acceptable cracks inadvertently reduces yield.
Further, vision techniques that rely on intensity are typically inadequate because the cracks 120, 122, 132 exhibit various intensity patterns. For instance, FIGS. 3A and 3B illustrate a cross-section of a deep crack 140 and its intensity histogram 142, which shows a change in intensity from an intermediate area 144 to a bright area 146 to a slightly darker area 148, then to the bright area 146 and finally back to the intermediate area 144. FIGS. 3E and 3F are an illustration of a cross-section bright crack 150 and its intensity histogram 152. Unlike the deep crack 140, the bright crack 150 does not have the slightly darker area 148. Not only can the pattern of intensities vary for cracks 120, but also the degree of intensity changes can vary as shown by FIGS. 3C, 3D, 3G, and 3H. A cross-section of a dull crack 170 and a sharp crack 160 have the same change in polarity from a light area 166, 176 to a dark area 168 and 178, but the change is not to the same extent, as illustrated by comparing the two intensity histograms 162 and 172.
The combination of varying intensity profiles of the cross-section of cracks, the textured surface of heads that provides an abundance of directional information, the presence of fairly similar sized cracks that are not of consequence, and the extraneous marks or non-uniformities has hindered, if not stopped, the automatic detection of the cracks.
Consequently, this process typically has been performed manually. However, manual inspection is very expensive.